求1-2+3-4+5 ... 99的所有数的和
# sum = 0
# count = 1
# while count < 100:
#     if count % 2 == 0:
#         sum = sum - count
#     else:
#         sum = sum + count
#     count += 1
# print(sum)


# sum=0
# for i in range(1,100):
#     if i%2==0:
#         sum=sum-i
#     else:
#         sum+=i
# print(sum)